Broad-scale patterns of genetic diversity and structure in a foundational salt marsh species black needlerush (Juncus roemerianus)

Clonal and genetic diversity in foundational plant species are critical for species resiliency and ecosystem processes, both of which contribute to restoration success; however, genetic data is often lacking for common plant species used in many restoration practices. Only a few plant species dominate salt marshes, ecologically and economically valuable ecosystems targeted for restoration due to global decline. Despite being a foundational species in southeastern United States salt marshes black needlerush (Juncus roemerianus Scheele) is understudied, especially in comparison to the co-occurring smooth cordgrass (Spartina alterniflora Loisel). We used a panel of 18 microsatellite markers on 849 samples of J. roemerianus collected at 17 sites across a majority of the species range from Mississippi to South Carolina to measure clonal and genetic diversity and characterize population structure. Results are consistent with previous genetic studies on J. roemerianus and other clonal plant species in that clonal and genetic diversity were higher than expected based on life history literature, with an average genotypic diversity (G(D)) of 0.67 and average observed heterozygosity (H-O) of 0.56. Differences in diversity between the ecologically and environmentally divergent Gulf (G(D)=0.64, H-O=0.52) and Atlantic (G(D)=0.80, H-O=0.45) coasts suggest that life history strategy may vary by environment. Hierarchical structure was apparent across the study area, with STRUCTURE analyses identifying three genetic clusters that further subdivided into five clusters and a broad transition zone. The scale of this genetic differentiation should be considered in sourcing plants for salt marsh restoration efforts.